The Franks acquire stirrups by about 730. They also develop exceptionally heavy horses (the breeds of northwest Europe are the ancestors of the carthorses later used for haulage and ploughing). With stirrups and a powerful horse, the medieval knight is ready to take the field.

A mounted knight in armour, usually of mail (also known as chain mail), is to a large extent protected from the archer's arrow or the spear of the footsoldier, while his own long lance is a lethal weapon against any opponent. Its thrust no longer depends on the strength of an arm. Seated in a shaped saddle, with his feet in stirrups and the lance held firm against his body, the knight drives home the point of the lance with the full forward impetus of his horse.

Both horse and armour are expensive, so warrior status is now reserved for the ruling class; and with faces concealed inside armour, devices on helmet and shield are essential to identify friend from foe. Painted armour happens also to be a glorious way of advertising one's lineage. It is no accident that possession of a 'coat of arms' is a distinguishing mark of European aristocracies.

Such a system of warfare is ideally suited to feudal societies. The mounted knight holds sway in Europe throughout the Middle Ages until new weapons in the 14th century - such as the pike and the longbow - restore some measure of advantage to the humble infantry.

Medieval castles: 9th - 13th century

In feudal Europe, where armed men are granted rights over often hostile territories, the castle becomes an important feature of the countryside. Such castles are often surprisingly flimsy affairs. It comes as a shock to read that William I, in his invasion of England in 1066, lands at Pevensey on September 28 and builds himself a castle before fighting the battle at Hastings on October 14.

It is of the mound-and-bailey variety, also called motte-and-bailey (from the Norman French motte for a mound). This is a design developed by the Franks in the 9th century and adopted by the Normans.

The construction of a mound-and-bailey castle is a simple matter of hard and rapid labour. A circular ditch is dug (when filled with water, it becomes a moat). The earth from it is piled inwards to form a mound, preferably adding height to an existing prominence. On top of the mound a tower is built, within a palisade.

An adjacent area is surrounded by another palisade, and sometimes also by a moat. This is the bailey, or outer courtyard, in which the garrison live and keep their livestock. A bridge crosses the moat to reach the more secure mound and its tower. In the first five years of the Norman conquest of England thirty-five such castles are established, nearly all of them of wood.

Where stone and time are available, it is clearly preferable to construct a castle of the stronger and non-combustible material. During the 12th century stone walls and towers become more common in European castles, together with more sophisticated forms of bastion and battlement.

One influence is the Byzantine castle architecture seen by the crusaders on their way east. They soon create in the Holy Land magnificently impressive examples of their own - such as the great Krak des Chevaliers, largely built by the Knights of St John and occupied by them from 1142.

In Europe the castle as a fortified garrison is seen in a highly developed form in the great series built in the late 13th century for Edward I along the coast of Wales, uncompromising in their purpose of keeping the Welsh in submission.

In subsequent centuries the castle evolves into something more akin to a great man's residence, his fortified palace. This is true of the famous French castles of the Loire, built in the 15th and 16th centuries. And it is true of the magnificent castles of exactly the same period in two very different cultures, in India and Japan.

Gunpowder: 10th century

In about 1040 a Chinese manual on warfare is issued under the title Compendium of Military Technology. It is the first document to describe gunpowder. This black powder, formed by pounding a mixture of saltpetre, charcoal and sulphur (a dangerous process if the pounding is overdone), seems to have been developed in the small chemical laboratories attached to the temples of Daoists where research is conducted mainly on the secret of eternal life.

At this early stage in China the military use of gunpowder is limited to grenades and bombs lobbed at the enemy from catapults. Its real destructive force will only emerge when the explosion is confined, in the development of artillery.

A weapon of mass destruction: 1139-1346

Pope Innocent II and the second Lateran council take a firm stand, in 1139, against a weapon which they consider morally unacceptable in its devastating capacity to kill. It is the crossbow, invented in China in the 3rd century BC and first recorded in use in Europe in a battle at Hjörungsvag in Norway in986.

The pope and his cardinals make one distinction in this early attempt at arms control. They specify that the weapon is unacceptable in warfare between Christians. They are pontificating a few decades after the success of the first crusade. The implication is clear. The weapon of death may be turned against Muslims.

The crossbow proves effective on crusade. Indeed Richard I wins one of his victories over Saladin, at Arsuf in 1191, largely because of the effect on the Muslim forces of the bolts fired by his crossbowmen. Each bolt, about ten inches long with a square metal head tapering to a point, can be shot with sufficient force to pierce contemporary armour at a range of up to 300 yards. The weapon's limitation is its very slow rate of fire.

The papal embargo fails to stop the spread of the crossbow among Christian armies. It is familiar on European battlefields in the 12th and 13th centuries. Mercenaries from Genoa in particular are famous as crossbowmen - until they confront a new and faster weapon at Crécy in 1346.

The strategy of the Mongols: 13th century

Several different factors explain the devastating success of Genghis Khan and his armies, but superior weaponry is not one of them. The traditional riding skill of the nomads of the steppes plays, as ever, a large part. With stirrups now a standard part of cavalry equipment, the agility of the horsemen is greater than ever, in galloping close to the the enemy, releasing a hail of arrows and wheeling away again.

Horsemanship also plays its part in the system of communication which enables Mongol armies to coordinate their strategies. Riders gallop between well-equipped staging posts across the steppes, enabling a message to travel more than 200 miles in a day. Pigeons, too, are trained for the purpose.

But the single most important element is a ruthless use of two psychological weapons, loyalty and fear. Genghis Khan makes a cunning distinction in his treatment of nomadic tribesmen and the settled inhabitants of cities and towns. A warrior from a rival tribe, who battles bravely against Genghis Khan but loses, will be rewarded for his valour and encouraged to join the Mongols against the rest of the world. Only cowardice or treachery in an opposing tribe are punished.

For sedentary folk in alien lands these rules are reversed. Here treachery is positively encouraged. Spies infiltrate the towns. Informers are sought out and bribed. The Mongols are coming. There is a choice to be made.

The choice is a simple one; to fight or to surrender. News of the consequences travels fast. If a town bravely resists, the inhabitants are massacred in a public display. They are herded outside the walls to confront Mongol troopers with battle-axes. Each trooper is given a quota to despatch. A tally of ears is sometimes demanded as proof that the work is done.

Terror stalks ahead of a Mongol horde like an invisible ally. The spies in the town let it be known that a rapid surrender may well be rewarded with mercy. Usually the citizens need no persuading. The gates are opened. After sufficient plunder to keep the troops happy, the horde moves on.